In vehicles using electric traction motors, alternating current (AC) motor drives are used to provide a requested torque to the motor shaft. In practice, the amount of torque produced by an electric motor is directly related (although not perfectly proportional) to the amount of current provided to the motor. Therefore, by regulating and precisely controlling the current to the electric motor, the amount of torque produced by the electric motor may be accurately controlled and monitored.
In many systems, the input motor current is not directly controlled. For example, many electric motors are operated using pulse-width modulation (PWM) techniques in combination with an inverter (or another switched-mode power supply) to control the voltage across the motor windings, which in turn, produces current in the motor. In response to a requested torque (or commanded torque), most prior art systems determine a desired input motor current for producing the requested amount of torque and utilize a closed loop control system to control the current through the motor windings and thereby regulate the amount of torque produced by the motor. Current sensors are used to measure the motor current, which is then compared to the desired input motor current. The PWM commands for the inverter are adjusted to increase and/or decrease the voltage across the motor windings, such that the measured motor current tracks the desired input motor current.
When a current sensor does not accurately measure the motor current, the ability of these closed-loop control systems to control the motor torque is impaired. For example, without accurate motor current information, the control system may cause the motor to produce insufficient torque, excessive torque, or varying or oscillating amounts of torque. Accordingly, it is desirable to monitor the current sensors and provide protection in the event a current sensor is not accurately measuring the motor current to ensure reliable operation of the electric motor.